1. Field of the Invention
The present invention relates to a receiver for receiving an OFDM (Orthogonal Frequency Division Multiple) transmit signal, and more particularly, it relates to improvement in the detection of a reception quality signal used particularly in antenna adjustment.
2. Description of the Related Art
Recently, a digital transmission system by use of an OFDM transmit signal has been put to practical use especially in a field of terrestrial digital broadcasting. The OFDM method acts to allocate data to a plurality of mutually orthogonal carriers (carrier waves) for modulation and demodulation, by which a transmission side conducts reverse FFT (Fast Fourier Transfer) processing and a reception side does FFT processing.
For each of the carriers transmitted by the OFDM method, an arbitrary modulation method can be employed such as a QAM (Quadrature Amplitude Modulation) method by means of coherent detection and a transmission method by means of differential detection. In coherent detection, a pilot symbol with an already known quality is periodically inserted beforehand so that the reception side may obtain a difference of a received signal with respect to the pilot symbol to thereby equalize the amplitude and the phase of the received signal. In differential detection, a received signal is differential-coded between the received symbols, to be demodulated without reproducing the carrier.
Terrestrial digital broadcasting presumes various reception conditions such as multi-path interference or interference from an existing analog broadcast, so that the magnitude of a reception power does not always indicates the reception quality. Therefore, antenna adjustment, for example, has a major object of permitting a receiver to detect a reception-quality signal at a high rate and also in a wide range.
The reception-quality signal detecting method is typically dependent on a bit error rate or an S/N ratio (variance value) of a demodulated signal.
The method by use of a bit error rate, however, has a problem that an attempt to detect a good reception state reduces errors in a received signal to thereby prolong a detection-unit time in which the number of samples required to detect a bit error rate is detected, thus spending rather a long time to detect the reception-quality signal. In the terrestrial digital broadcasting by use of the OFDM transmission system, as the modulation method is prescribed a QPSK (Quadrature Phase Shift Keying), 16QAM, or 64QAM method, so that if a reception-quality signal is detected simply using an error rate or an S/N ratio of demodulated data, the modulation method employed may restrict the detection range problematically. For example, the 64QAM method gives a small interval between the adjacent demodulated signals as compared to that by the QPSK method and so finds it difficult to detect the demodulated signals if they have a large scatter. That is, the 64QAM modulation method gives a narrow detection range of a reception-quality signal as compared to the QPSK method.